This disclosure relates generally to the field of radiation monitoring and dosimetry, and more particularly to detection of neutrons, including thermal neutrons, by a solid state radiation detector.
Radiation may come in various forms, including x-ray, γ-ray, β-ray or α-particle emission. There are many types of radiation monitors that may be used to determine an amount of radiation exposure, such as ionization detectors, Geiger counters, and thermoluminescent detectors (TLDs). Geiger counters and ionization detectors may determine and display a dose rate (for example, in mRad/hr) or an integrated dose (for example, in Rads) of radiation exposure in real time. Alarm set points may be programmed based on the dose rate or the integrated dose. A Geiger counter or ionization detector may communicate with a computer for data logging or firmware updates. However, Geiger counters and ionization detectors may be relatively expensive. TLDs allow determination of a dose of radiation based on emission of photons in response to application of heat. TLDs may be relatively inexpensive, but may only be read after a period of exposure time, typically between one and three months. A degree of radiation exposure may only be determined after-the-fact using a TLD; real time dose information is not available.
A semiconductor, or solid state, radiation monitoring device may comprise a metal-oxide-semiconductor field effect transistor (MOSFET) transistor structure having a gate oxide layer fabricated on bulk silicon. A charge is induced in the FET structure by ionizing radiation exposure and trapped in the gate oxide of the FET by a voltage applied to the gate. The threshold voltage (Vth) of the FET may change according to the amount of trapped holes. A dose of radiation experienced by the solid state radiation monitoring device may be determined by determination of the change in Vth.
A FET radiation detector may be fabricated using a fully depleted silicon-on-insulator (FDSOI) FET device that is capable of detecting doses of various types of ionizing radiation, and that exhibits long-term charge retention that enables long-term tracking of total radiation dosage. The FDSOI radiation detector may be made as small or large as desired using semiconductor wafer fabrication technology, and may have a relatively low power drain. A charge may be induced in a buried oxide (BOX) layer of the FDSOI radiation detector by radiation exposure and trapped by voltage applied to a back gate contact or body contact. Determination of the amount of induced charge through determination of the Vth is then used to determine an amount of radiation exposure experienced by the FDSOI radiation detector. An example of an FDSOI radiation detector is found in U.S. patent application Ser. No. 12/719,962 (Gordon et al.), filed Mar. 9, 2010, assigned to International Business Machines Corporation, which is herein incorporated by reference in its entirety.
While radiation including charged particles such as alpha particles (α), protons (p), and electrons (e), or neutral particles such as x-rays and γ-rays may be detected using a FDSOI radiation detector, neutron detection is more difficult, particularly detection of thermal neutrons. A thermal neutron is a neutron having a relatively slow speed and consequently low energy, and that may only travel a relatively short distance in silicon. The energy (E) of a thermal neutron may be on the same order as the thermal energy of the atoms or molecules of a substance (such as air) through which the thermal neutrons are passing; i.e., about 0.025 electron volts (eV). Thermal neutrons are responsible for various types of nuclear reactions, including nuclear fission. Thermal neutron detection is important as higher-energy neutrons from such radiation sources as weapons of mass destruction (WMD), improvised nuclear devices (IMD), or the detonation of nuclear bombs become thermalized as the neutrons pass through air or other materials.